1. Field of the Invention
The invention relates to an electrohydraulic control system for controlling gearspeed changes in partially or fully automatic transmissions having at least one gear changing element that can be activated hydraulically, a control unit with a signal input from at least one sensor, and a signal output for electrically actuating at least one value in order to activate a gear changing element.
2. Description of the Related Art
It is generally known to control partially or fully automatic transmissions electrohydraulically. In electrohydraulic transmission controllers it is possible to distinguish between systems which have a separate pressure modulation levelxe2x80x94i.e. the working pressure for the gear-changing elements is generated in an oil circuit and the control slide which conducts this working pressure to the gear-changing element is shifted by means of a further control level generally with a lower oil pressure level via a solenoid valvexe2x80x94and systems which have direct hydraulic actuation of the gear-changing elements.
The uncertain variable in an automatic gear-changing operation is the dead time between a gear-changing command and the effect on the gear-changing element, which effect is dependent on the air play, i.e. on the distance between the force-transmitting elements when they are in the opened statexe2x80x94the wear state, the installation tolerances etc. This uncertainty can be significantly reduced by pre-charging the clutch and measuring the working pressure, thus improving the response behaviour of the clutch. As a result of direct electrohydraulic actuation of each gear-changing element, the gear-changing behaviour can be influenced most effectively and most flexibly, but when an electrical unit fails, all the gear-changing elements must be deactivated in order to protect the occupants of the vehicle and the transmission or the gear-changing elements of a certain gearspeed must be activated so that a gearspeed change is no longer possible.
Taking this as a starting point, the object of the invention is to provide an electrohydraulic control system for fully or partially automated transmissions, which permits gear-changing operations in an emergency mode when an electrical or electronic element fails.
This object is achieved according to the invention by means of an emergency gear change slide which can switch off the electrical actuation of the valves so that the valves can be controlled hydraulically.
In one control system according to the invention, all the regulating magnets are switched off when there is a power failure or when there are other errors in the electronics. The regulating valve slide can then advantageously be hydraulically modulated with an emergency gear change device. This is then a case of passive redundancy with an electrical xe2x80x9cshift-by-wirexe2x80x9d main pathxe2x80x94i.e. in the normal mode gearspeed changes are controlled by means of electrical signalsxe2x80x94and a mechanical/hydraulic secondary path which is activated only when the first fails. The selected approach can be used to implement operations ranging from the maintenance of the gear last engaged when there are electrical errors, to hydraulic locking operations such as correspond to the prior art in military caterpillar vehicles.
The emergency gear-changing operation makes it possible in a particularly advantageous way to change the gear of the transmission temporarily when there is a failure of the supply voltage, a defect in the electronic controller or some other electrical fault.
The control system according to the invention is particularly flexible and can easily be adapted to different types of transmission.
Further features and advantages emerge from the subclaims in conjunction with the description.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.